Plastic Bag Dust Collector System

A woodworker develops his concept for a dust collection system pairing filter bags with plastic collection bags. August 3, 2009

QuestionI'm thinking about upgrading my dust collection system. I currently blow all my shavings directly into a small box trailer. The system is made up of a 5hp blower outputting directly to the trailer (which is located behind my building). This works well in terms of not having to deal with shavings and filtration bags, but of course draws the air out of the shop. In the summer this draws in hot air and in the winter draws out the heat and pulls in outside cold air. Of course there's the shavings mess around the trailer from blow-by.

I'm thinking it's time to set up a bag collection system, sort of a mini baghouse. I want to keep the existing blower and exit pipe. Instead of directing it to the trailer though I would direct it to the filter and collection bags. I have a good semi-enclosed area on the side of the building that will work well for this purpose. I can completely close it in and make a furnace style filtered opening back into the main shop for the air return.

I'm picturing something that looks like the collectors with the fabric filter bags extending above an inlet manifold, and perhaps 30-50 gallon range clear plastic bags below collecting the shavings. There would be a line of perhaps four collection bags with filter bags above. These units can of course be purchased complete with blowers, but I want to keep the existing blower in place. So, can anyone give me guidance on this type of setup? I have an additional limitation of ceiling height though that is around 7 1/2 to 8 feet max.

Forum Responses(Dust Collection and Safety Equipment Forum)
From contributor J:I believe you if you say such arrangements are commercially available, but it seems doubtful that plastic bags would last long under the pressure of a 5hp blower. I'm more familiar with smaller systems, and the ones I've seen have fabric bags above and below. What about mounting a cyclone with one of those motorized vacuum-lock thingies (the proper term escapes me) outside, over the trailer, so you don't have to futz with emptying the bags all the time, and ducting the exhaust back into the shop, with only a fine filter (bag or pleated) inside?

From contributor T:There are a number of manufacturers that make these systems with the plastic bags. We just moved facilities and went with a large external DC, but at the old shop we were using two 7.5hp systems. A couple of pointers - the bags are not re-useable unless you like cleaning up messes, and don't buy the bags from the manufacturer, you'll get lower quality at four to five times the price you can get bags made for. We used Polypak plastics that custom made the bags for us out of 5mil poly, no problems. You can also pay more for a bladder that fits over a 50gal reusable drum, the only problem is you canít tell when its full.
From contributor F:I've been using Husky bags from the orange box store for the last couple of years on all my little 1-1/2 and 2hp collectors and they have been better than the ones provided with some collectors. Iím not sure how they would work with a larger 5 hp motor, but then again you'll have four times the filter and collection area. I also found out the hard way not to re-use the bags the manufacturers provide, talk about a messy situation.

Another thing you should at least look into would be canister filters. I put one on one of my units and it makes a drastic difference in suction, as well as taking up less room height wise. You have an advantage as there are far fewer shops looking for the bigger 3-4 bag units. My smaller unit days are almost over though. I just picked up a 7-1/2 hp Torit cyclone with 3 stage after-filter. I expect to be breathing a lot less dust in the near future.

From contributor D:I think you might want to give the plastic bag shavings capture some more thought. While it is mostly dependent upon how many shavings you produce, removing and dumping full plastic bags and remounting empty ones not only take a lot of time, but also expose you (someone) to the fine dust. This is also overhead, since you can't bill for dumping shavings. I bought my first small shop central collection (Oneida) after I added up all the time I spent emptying dust bags. I realized I had spent the dollars, but didn't have the system. I ordered one the next day.

The first system blew all the shavings through a cyclone and then they dropped into a 4' x 4' x 7' airtight room, with the return air going to the shop thru filter socks in a manifold, tucked up in the gable of the shop. The airtight room is accessed from the outside, and the shavings flow out the opened door at pickup truck height.

The next shop had a much larger airtight room, with an auger at the bottom to convey the shavings up into a dumpster. The airtight room was hardly airtight, and required constant resealing to keep up the pressure. If the pressure dropped too much, the dust and shavings did not fall into the airtight room but passed into the filter socks, filing them - no fun.

In our new shop, we are done with the airtight enclosures and will use a rotary airlock positioned over an eight yard dumpster. The airlock will dribble shavings into the dumpster by gravity, and the dust will be minimized. The filter socks will all be reused, and the system will maintain pressure due to the rotary airlock.

From the original questioner:To contributor D: you didn't mention cleaning the filter socks. I assume that needs to be done periodically? Is there no blow-by dust arriving at the socks that needs to be captured in some sort of collector?
From contributor D:We tapped on the bags about once a week to knock off excess filter cake. We were told that some filter cake is needed for the best filtration. We have never cleaned the bags before, but do blow off the outside occasionally. Our setup is similar to a baghouse, without the house. We had the bags attached to a lower airtight plywood box that caught the fine dust. This box needed to be emptied periodically - more frequent if the system was not airtight. This next configuration has the bags sealed to a platform with a removable floor, located all above a second eight yard container. While we are expecting to have to clean out once or twice a year, it will be easier since we only have to unseal the floor and drop the fine dust out. The CFM of the blower is tied to the s/f of the bag surface - this is where an engineer/experienced individual can help you determine material and s/f. You can have low profile, as long as you have the s/f to accommodate the CFM you will move. I saw one setup, 20 bags long, one wide, to fit in an otherwise unusable space.
From contributor F:To contributor D: I'm curious about your system. So the rotary airlock essentially catches the dust at the bottom of the cyclone and pushes it out into another container, and this container does not need to be airtight?

If so I may have to look into adding one to my system. My biggest concern is emptying (and keeping airtight) the 50 gallon drum that comes with it constantly. Did Oneida help you with the airlock configuration or did someone else?

From contributor D:Yes, the rotary airlock is bolted onto the bottom of the cyclone, and keeps pressure in the system, but dribbles out the shavings with a four vane rotating shaft that keeps it all sealed above. Koger is the big manufacturer - on the web.

The RAL is an expensive bugger, heavy, and has its own motor. But this way we do not have to have the receptacle airtight. The shavings drop at a slow speed into the dumpster. We think the low velocity and no pressure will be clean and dust free. If we have to, we can enclose with walls and a tarp cover. I got the RAL through Oneida, and they have tech help. The RAL is little known in smaller shops, and with its expense, little used. It can eliminate the need for airtight enclosures. The nicely engineered system running smoothly but dumping into a 55 gal drum was all wrong in my mind. We'd spend way too much time moving drums around - then you have to dump them, right?

From contributor L:We've got a Koger rotary air lock on the cyclone and use a pull-through system with a reverse inclined blower. These blowers are more efficient than material handling blowers. Our bag house was bought used, cheap ($350) and has an automatic shaker, 48 filter bags 6" x 72" (about $7 each.) Everything is outside but we can return the heated air to the shop. The air lock drops through a chute with a window into an enclosed dump trailer. Only extremely fine powder goes into the baghouse (unless someone lets the trailer over fill, then itís a mess)! We have blowout panels on the bag house and the return air plenum should there be an explosion. We need to upgrade and move our collector system and do something to distribute the shavings in the trailer.

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